Remember this date, it is highly important.
I am now going to tell you about that curious phrase.

On the evening before the crime, that is to say, on the 23rd, Monsieur and Mademoiselle Stangerson were at a reception at the Elysee.

I know that, because I was there on duty, having to interview one of the savants of the Academy of Philadelphia, who was being feted there.

I had never before seen either Monsieur or Mademoiselle Stangerson.

I was seated in the room which precedes the Salon des Ambassadeurs, and, tired of being jostled by so many noble personages, I had fallen into a vague reverie, when I scented near me the perfume of the lady in black.
"Do you ask me what is the 'perfume of the lady in black'?
It must suffice for you to know that it is a perfume of which I am very fond, because it was that of a lady who had been very kind to me in my childhood,--a lady whom I had always seen dressed in black.